ImGui::IsNamedKey(key) error on Startup

### Describe the bug

When launching PCSX Redux, the following error appears and hard closes PCSX Redux on OK.
This happened after using the auto updater.

```
---------------------------
Error
---------------------------
ImGui assert: IsNamedKey(key) && "Support for user key indices was dropped in favor of ImGuiKey. Please update backend & user code." is false
---------------------------
OK
---------------------------
```

I've re-generated `pcsx.json` in Roaming data and that has fixed it, for anyone else with this issue.

I've attached my broken version below, with paths removed.

### Expected behavior

PCSX Redux opens as normal.

### Steps to reproduce the bug

1. Open PCSX Redux with following JSON.
[pcsx.old.json](https://github.com/user-attachments/files/31883007/pcsx.old.json)

2. See error marked above.

### Operating System

Windows 11

### PCSX-Redux version

25270.20260904.6.x64
Build 384

### CPU model

AMD Ryzen 9 7950X 3D

### GPU model & Drivers

Geforce RTX 5080

### BIOS version

OpenBIOS

### Options

- [ ] Dynarec CPU
- [ ] 8MB
- [ ] OpenGL GPU
- [ ] Fastboot
- [ ] Debugger

### Iso checks

_No response_

### Logs

_No response_

### Additional information

_No response_

Start by reproducing the startup failure on Windows 11 with the attached pcsx.old.json, then inspect the startup configuration loading and ImGui key handling. Compare the broken configuration with a regenerated pcsx.json; done means PCSX Redux opens normally without the assertion while handling the affected configuration.

Written by the indexing model from the issue text.